Electrical resistance element



, p w. L. GREENWOOD I 3,343,115

ELECTRICAL RESISTANCE ELEMENT Filed Dec 2, 1964 5 Sheets-Sheet 1 FIG. 1

INVENTOR. WHITNEY L. GREENWOOD ATTORNEY Sept. 19, 1967 w. L. GREENWOOD 4ELECTRICAL RESISTANCE ELEMENT Filed Dec. 2, 1964 5 Sheets-Sheet 2 7 FIG.4 INVENTOR.

WHITNEY L. GREENWOOD BYQA%'%74A Sept. 19, 1967 w. L. GREENWOOD 3,343,115

I ELECTRICAL RESISTANCE ELEMENT Filed Dec. 2, 1964 3 Sheets-Sheet s 62 LINVENTOR. WHITNEY L. GREENWOOD BY H6. 6 $7 ATTORN United States Patent3,343 115 ELECTRICAL RESISTANCE ELEMENT Whitney L. Greenwood, La Habra,Calif., assignor to Beckman Instruments, Inc., a corporation ofCalifornia Filed Dec. 2, 1964, Ser. No. 415,351 7 Claims. (Cl. 338-174)ABSTRACT OF THE DISCLOSURE A variable resistance device including aresistance element having ends thereof terminating adjacent anonconductive region, a collector element having ends spaced from butassociated with the respective ends of the resistance element and aconductive Wiper adapted to simultaneously traverse the resistance andcollector elements, the ends of the collector element being so arrangedwith respect to the respective ends of the resistance element and thecontacts of the Wiper as to electrically disconnect the wiper fromexternal electrical circuitry at a point in wiper travel prior to theinstant the wiper traverses from the resistance element to thenonconductive region.

The present invention relates generally to an electrical impedance orresistance element and, more particularly, to an electrical resistanceelement of the type used in a variable resistance device having amovable contact or Wiper adapted to traverse the resistance element.

The electrical resistance element employed in variable resistancedevice, such as a potentiometer, rheostat, and the like, may be arrangedin a variety of ways. For example, the element may be disposedrectilinearily, in which case the Wiper moves generally in a straightline along the element, or it may be disposed in a circular fashion, inwhich case the wiper rotates about a centrally located axis.Irrespective of the physical form or disposition of the resistanceelement, the wiper may be required to leave the electrically activeportion of the resistance element and traverse an insulated bridge ordead space. When the resistance element is formed in a circular shape,for example, an insulated bridge or dead space must of necessity beprovided between the end points of the element to prevent ashort-circuit of the potential diiference existing between the two endpoints during operation of the device. In certain applications it may berequired that the wiper of a variable resistor employing a circularresistance element be continuously rotated, in which case the wipertraverses the dead space each revolution.

In formed-base variable resistance devices, which employ resistanceelements made of a resistance material which is applied as a thin layeror film on a nonconductive support or base, certain deleterious effectsare produced when, during sustained operation of the device, the movablewiper is caused to continually leave an end point of the resistanceelement. For example, burning of the end points of the resistanceelement has been observed. This may change the electrical resistivity ofthe element at those points and may also change the Wiper break-offpoint, that is, the point at which the wiper breaks contact with theelectrically active portion of the resistance element. Burning of theelement at its end points adversely affects the service life of thedevice and appears to result from the structural characteristics of thefilm or layer type resistance elements. To aid in the understanding ofthe cause of these problems, some discussion regarding the apparentstructural nature of the subject resistance materials may be helpful.

One such material, commonly called cermet is formed of a ceramic-typebinder, such as glass, having dispersed therein finely divided particlesof metal. See for appear to indicate that the dispersion of the metalparticles in the ceramic binder may not be uniform throughout allportions of the resistive layer. As a result, at the end points of theresistance element, there may be concentrations of only a few metalparticles separated laterally (that is, across the width of theelement), by relatively large interspaces of nonconducting binder. Sinceall the current flowing between the Wiper and resistance element mustpass through the few metal particles at the end of the resistanceelement, the current concentrations may be of such magnitude as to causemelting and oxidation of the metal particles. It further appears thatthe temperatures generated may be sufficient to momentarily melt theglass binder in the immediate vicinity of the metal particles. Uponsolidification, isolated islands of fused metal particles may be formedwith a resultant loss in continuity coupled with a shift in the wiperbreak-off point. Additionally, the electrical resistivity at the burnedend point of the resistance element may also be altered.

If the current drawn by the wiper is large enough at the moment thewiper leaves the resistance element end point, another problem which mayarise is arcing between the Wiper and end point. Arcing causes localizedhigh temperatures which may result in further deterioration of theelement end points due to burning and erosion.

The problems delineated above, and which the present invention seeks toovercome, arise in a similar fashion in other types of resistanceelements such as those fabricated of conductive plastic in whichparticles of graphite or other conductive material are dispersed in anorganic (nonconducting) binder. Thus, it is intended that the inventiondescribed below and shown in the drawings be equally applicable to allresistance devices employing resistance elements of the classexemplified by cermet and conductive plastic.

It is therefore an object of this invention to provide a variableresistance device in which the end points of the resistance element arenot subjected to burning, arcing or other detrimental phenomena.

'It is a further object of this invention to provide a variableresistance device with a switching means to protect the end points ofthe resistance element from burning, arcing and other detrimentalphenomena.

It is a more specific object of this invention to provide means'forinterrupting the flow of current through the wiper before the wiperleaves the resistance element and for reestablishing current flowthrough the wiper after the wiper has traversed the insulated bridge ordead space and is again in contact with the resistance element.

According to a first, specific, exemplary embodiment of the invention,there is provided a variable resistance device comprising anonconductive support or base having a surface upon which a film orlayer of resistance material such as cermet or conductive plastic hasbeen applied or otherwise atfixed in the form of a first track of agiven length adapted to be traversed by a movable wiper. If the track isformed in a circle, an insulated bridge or dead space comprising a smallsegment of the circle is provided between the end points of the track. Asecond film track made of a highly conductive material such as gold orsilver and extending parallel to or concentric with the first track isapplied to the conductive base and serves as a commutator or collectorfor electrically connecting the wiper member with an external circuit.The end points of the resistance element comprising the first track aredisplaced with respect to the end points of the collector elementcomprising the second track in such a manner that the wiper breakscontact with the collector element (and hence the external circuit)before it breaks contact with the resistance element. As a result, thewiper is no longer drawing current by the time it leaves the resistanceelement and begins traversing the insulated bridge space separating theresistance element ends. Burning due to current concentrations at theresistance element end points is there'- fore eliminated and any arcingwhich may occur will take place between the wiper and the collectorelement which, because of its high conductivity, is capable ofdissipating large quantities of heat.

According to another, specific, exemplary embodiment of the invention,there is provided a variable resistance device in which thecorresponding end points of both the resistance element and thecollector element are in alignment. However, the wiper contacts whichslidably traverse the collector element are made smaller than the wipercontacts which slidably traverse the resistance element. As a result,the connection with the external circuit is broken first at one or theother end point of the collector element since the collector wipercontacts leave the end point of the collector element before the largerresistance element wiper contacts leave the corresponding end point ofthe resistance element. As in the first exemplary embodiment described,the detrimental effects of high current concentration and arcing at theresistance element end points are eliminated.

The invention, together with further objects and ad vantages thereof,can best be understood by reference to the following description takenin conjunction with the accompanying drawings in which:

FIG. 1 represents, diagrammatically, the circuit of a variableresistance device incorporating a first embodiment of the presentinvention;

FIG. 2 is a perspective view of a rectilinear motion variable resistancedevice incorporating the first embodiment of the present invention;

FIG. 3 is a perspective view of a rotary motion variable resistancedevice incorporating the first embodiment of this invention;

FIG. 4 is a plan view of a portion of the rotary motion variableresistance device depicted in FIG. 3;

FIG. 5 is a perspective view of a portion of a rectilinear motionvariable resistance device incorporating a second embodiment of theinvention; and

FIG. 6 is a bottom view of the wiper assembly of the variable resistancedevice of FIG. 5.

Referring now to the drawings in which like reference numerals designatelike elements, FIG. 1 illustrates a schematic representation of avariable resistance device connected, for exemplary purposes, as avoltage divider. The device comprises a resistance element 10 having endpoints 12 and 14 across which may be connected a voltage source orinput, e Placed immediately adjacent element 10 and running generallyparallel therewith, is a commutator or collector element 16 having endpoints 18 and 20. Collector 16 connects the resistance element 10 withan external output circuit through wiper 22. Wiper 22 is adapted to movehorizontally in the device of FIG. 1 and thereby traverses both theresistance element 10 and the collector element 16. The wiper is inslidable, electrical contact with both of these elements throughout theportion of its travel between end points 18 and 20 of the collector 16.The output voltage e of the voltage divider appears across end points 14and 20 of elements 10 and 16, respectively. The output voltage e is afunction of the input voltage e the position of the wiper 22 along theresistance element 10 between end points 18 and 20 of the collector 16and the output circuit load (not shown).

As shown in FIG. 1, the left-hand end point 12 of resistance element 10is horizontally displaced a distance d from left-hand end point 18 ofthe collector 16. Righthand end points 14 and 20 of the resistanceelement and collector element, respectively, are likewise displaced. Itis thus aparent that if the wiper 22 is moved toward either the left orthe right, electrical contact between the wiper 22 and the collectorelement 16 will be broken first. For example, if the wiper is moved tothe left from initial position x to position y and then to position z,the latter two positions being shown by the dotted representations inFIG. 1, it may be seen that the wiper initially breaks contact with thecollector 16 at end point 18 and later with the resistance element 10 atend point 12. Hence, the wiper breaks the electrical connection betweenthe resistance element and the external circuit at point.18 therebyprotecting the resistance element at point 12 from burning. Further,assuming sufficient current is being drawn by the wiper at the instantof separation, arcing only occurs between the wiper and end point 18 ofthe collector.

As is evident from FIG. 1, if the input voltage e, is applied across thewhole length of the resistance element 10 at points 12 and 14, only thatportion of the resistance element lying between end points 18 and 20 ofthe collector will be active. For example, if the end portions of theresistance element designated by the distance d each represent 5% of thetotal resistance, then the usable resistance range will be 5% to Theoutput voltage e will be similarly limited with respect to the inputvoltage e;. If a resistance range of' essentially 0% to is desired, theinput voltage e may be connected across intermediate points 24 and 26instead of end points 12 and 14. It is desirable to have intermediateconnection points 24 and 26 lie opposite end points 18 and 20,respectively, of the collector. However, since that configuration mayresult in discontinuities in the output voltage when the wipersimultaneously traverses an intermediate connection point and thecorresponding end point of the collector element, the intermediateconnection points 24'and 26 may be located a short distance inside thecorresponding collector element end points. This configuration is shownin FIG. 1 and assures a smooth 0%100% output curve, although the activeor effective length of the resistance element is somewhat shortened. Theeffective length of the resistance element, lying between points 24 and26, may represent, for example, 86% of the total length of the elementbetween the end points 12 and 14. The output voltage e in this case isobtained across points 26 and 20 of the resistance element andcollector, respectively, as shown by the dotted lines.

FIG. 2 of the drawings shows a rectilinear motion variable resistancedevice constructed in accordance with the inventive concept describedabove with the aid of FIG. 1. Resistance element 10 with end points 12and 14 and collector element 16 with end points 18 and 20 are affixed toan insulating base 28. Resistance element 10 may be a deposited cermetfilm, a molded conductive plastic, or other suitable resistive material.Collector element 16 typically may be a deposited film of gold or silveror other highly conductive substance. Terminals 30 and 32 are shownconnected to the resistance element 10 at its end points 12 and 14.Alternatively, terminals 34 and 36 may be connected to intermediatepoints 24 and 26 on the resistance element to obtain substantially a 0%-100% range. Points 24 and 26 lie short distances inside points 18 and20, respectively. Terminal 38, connected to the collector 16, isprovided for connection to the external circuit. Wiper assembly 22, madeof a suitable conducting material, is shown in sliding contact with bothelements 10 and 16. The wiper assembly 22 is aflixed to a traveling nut40 which is internally threaded for engagement with a threaded shaft 42.Rotation of the threaded shaft 42 in either direction by en externalforce apply-ing means (not shown), causes the traveling nut 40 to movelongitudinally in one direction or the other along the threaded shaft42. The wiper 22 may thereby traverse the entire length of theinsulating base 28.

The application of this invention to a rotary variable resistance deviceis shown in FIGS. 3 and 4. In this embodirnent, an insulating base 28 isshaped in the form of a circular disc with resistance element 10 andcollector element 16 applied in ringlike fashion to the surface of thedisc and generally concentric therewith. Although the drawings depictthe resistance element 10 as being placed exteriorly of the collectorelement 16, the positions of these elements, if desired, may bereversed. The wiper 22, in slidable contact with elements 10 and 16, isattached to insulated collar 44 which in turn is fixed to a rotary driveshaft 46. Rotary drive shaft 46 is generally coaxial with the resistanceelement 10 and collector 16. Rotation of shaft 46 thus causes the wiper22 to rotatably traverse the elements 10 and 16. End points 12 and 14,which lie on radii extending from the drive shaft axis, are separated byan insulating bridge 48 made of a nonconductive material such as glassor plastic or other material dependent upon the type of resistanceelement employed. The surface of the bridge 48 should be flush with thesurface of the resistance element on either side of the bridge so thatthe wiper will make a smooth transition from the resistance element tothe bridge and back to the resistance element. Similarly, a circular arcsegment comprising bridge 50 is provided between end points 18 and 20 ofthe collector element 16. End points 18 and 20 also lie on radiiextending from the'drive shaft axis. Bridge 50 may be fabricated ofappropriate insulating material such as fused glass or a thermosettingplastic, the surface of the bridge 50 being flush with the adjacentsurfaces of the collector element 16 to insure unhindered travel of thewiper 22. Terminal strips 52 and 54, which may consist of conductivesilver paint fused on the resistance element or metal strips embedded inthe resistance element or the like, serve to connect the resistanceelement 10 with external circuitry. Likewise, a metallic conductor baror an underlaid strip 56 of conductive silver paint is provided as acircuit connection for the collector element 16.

As in the rectilinear embodiment of the variable resistance devicedepicted in FIG. 2, the resistance element terminals may alternativelybe connected at intermediate points 24 and 26 to provide essentially a%-100% resistance range. Such alternate connection for the rotaryvariable resistor is illustrated in FIG. 4.

Bridges 48 and 50 may be placed symmetrically of a radial line 58.Collector bridge 50 includes an angle slightly larger than resistanceelement bridge 48 in order to achieve the objectives of the invention.By way of example, bridge 48 may extend on each side of the line ofsymmetry 58 while bridge 50 may be 6 on each side. Accordingly, thedifference angle, designated in FIGS. 3 and 4 as I9, is then 1.

Referring to FIGS. 3 and 4, it can be seen that in operation of therotary variable resistor, the wiper, whose contact fingers lie along aradial line, breaks contact first with the collector element 16 (andhence, the external circuit) irrespective of the direction of rotationof the wiper.

FIGS. 5 and 6 show an alternative embodiment of the invention as appliedto a rectilinear motion variable resistance device. Referring to FIG. 5,a wiper 22, with resilient contact fingers 60 and block-like wipercontact elements 62 and 64, is caused to longitudinally traverse, ineither direction, the length of the resistance and collector elements 10and 16, respectively, by an actuating mechanism such as that depicted inFIG. 2.

In this embodiment of the invention, the corresponding end points 12, 18and 14, '20 of the resistance element 10 and the collector element 16are in alignment. Thus, in the case of the rectilinear motion deviceillus- 64 and the collector element 16. The ends of the resistanceelement are thereby protected from burning, arcing and other adverseefiiects.

It will be obvious that this alternative embodiment of the invention isequally applicable to variable resistance devices having forms otherthan the rectilinear device of FIGS. 5 and 6. For example, a rotaryvariable resistance device, constructed in accordance with thealternative embodiment of the invention, would comprise resistance andcollect-or elements whose corresponding end points lie substantially onthe same radius. The collector element contacts of the wiper wouldinclude a smaller angle than the resistance element contacts, the lattercontacts overhanging the former by substantially equal angular distances0 on either side so that the benefits of the invention may be realizedirrespective of the direction of rotation.

It will be equally obvious that a 0%-l00% resistance range may beachieved in the alternative embodiment if the resistance elementterminals are attached to the resistance element a distance somewhatgreater than d (or angular distance 6, in the case of the rotaryvariable resistance device) from the end points.

While there has been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made therein without departure from theinvention and therefore, it is intended to cover all such changes andmodifications as fall within the true spirit and scope of the invention.I

What is claimed is:

1. A resistance device comprising a noncond-ucting base member;

a resistance element supported by said base member;

a collector element supported by said base member;

said resistance element being longer than said collector element, eachof said elements having end points so arranged that said end points ofsaid resistance element extend beyond said end points of said collectorelement;

electrical terminals connected to said resistance element at pointslying intermediate the end points of said collector element; and

a movable wiper traversing 'in slidable contact both said resistanceelement and said collector element during part of its travel but onlysaid resistance element during another -part of its travel.

2. The resistance device of claim 1 wherein said resistance element isin the form of a first film track supported by said base member; and

said collector element is in the form of a second film track supportedby said base member.

3. The resistance device of claim 2 in which said first track is spacedequidistant from said second track.

4. In a resistance device having a nonconducting base member, aresistance element with end points, a collector element with end points,said resistance and collector elements being supported by said basemember, and a wiper element adapted to traverse, in slidable contact,both said resistance and collector elements, the improvement comprisingthe end points of the resistance element being located beyond the endpoints of the collector element in the direction of wiper travel asufiicient distance to allow the wiper element to leave an end point ofthe collector element before it leaves the corresponding end point ofthe resistance element electrical terminals connected to said resistanceelement at points lying intermediate the end points of the collectorelement.

5. A resistance device comprising a noncond-ucting base member;

a resistance element in the form of a first track of a first givenlength supported by said base member,

a collector element in the form of a second track of a second givenlength supported by said base member;

said first track being spaced substantially equidistant from said secondtrack;

at least one end of said first track extending beyond the correspondingend of said second track;

electrical terminals connected to said first track at points lyingintermediate the end points of said second track; and

a movable wiper traversing in slidable contact both first and secondtracks during part of its travel but only said first track duringanother part of its travel.

6. A variable resistance device comprising a nonconducting base member;

a resistance element in the form of a first linear track, having a firstlength, supported by said base member;

a collector element in the form of a second linear track,

having a second length, supported by said base mem- 1 in the form of afirst circular arc segment of a first included angle;

the end points of said circular resistance element being separated by afirst insulated bridge element comprising a circular arc segment lyingbetween the end points of said circular resistance element;

said resistance element and said first bridge element together forming afirst circular track;

a collector element supported by said base member in the form of asecond circular arc segment of a second included angle;

the end points of said circular collector element being separated by asecond insulated bridge element comprising a circular arc segment lyingbetween the end points of said circular collector element;

said collector element and said second bridge element forming a secondcircular track;

said second circular track being concentric with said first circulartrack, and said second included angle of said collector element beingsmaller than, and lying within, said first included angle of saidresistance element;

electrical terminals connected to said resistance element at a pointlying intermediate the end points of said collector element; and

a rotatable, conductive wiper element in sliding contact with both saidresistance and collector element tracks during part of its travel butonly with said resistance element track during another part of itstravel.

References Cited UNITED STATES PATENTS 2,873,336 2/1959 Tassara 338-474X 2,959,729 11/1960 Karg 338l30 X 3,174,000 3/1965 Goldbeck 200-163,206,702 9/1965 Greenwood 338162 RICHARD M. WOOD, Primary Examiner.

J. G. SMITH, Assistant Examiner.

1. A RESISTANCE DEVICE COMPRISING A NONCONDUCTING BASE MEMBER; ARESISTANCE ELEMENT SUPPORTED BY SAID BASE MEMBER; A COLLECTOR ELEMENTSUPORTED BY SAID BASE MEMBER; SAID RESISTANCE ELEMENT BEING LONGER THANSAID COLLECTOR ELEMENT, EACH OF SAID ELEMENTS HAVING END POINTS SOARRANGED THAT SAID END POINTS OF SAID RESISTANCE ELEMENT EXTEND BEYONDSAID END POINTS OF SAID COLLECTOR ELEMENT; ELECTRICAL TERMINALSCONNECTED TO SAID RESISTANCE ELEMENT AT POINTS LYING INTERMEDIATE THEEND POINTS OF SAID COLLECTOR ELEMENT; AND A MOVABLE WIPER TRAVERSING INSLIDABLE CONTACT BOTH SAID RESISTANCE ELEMENT AND SAID COLLECTOR ELEMENTDURING PART OF ITS TRAVEL BUT ONLY SAID RESISTANCE ELEMENT DURINGANOTHER PART OF ITS TRAVEL.